When searching several databases, it is possible that you will retrieve the same item in your result set multiple times. Merge Duplicates allows you to collapse duplicates into a single search result.
ZPORTAL tells you how many hits have been returned (i.e. displayed) and how many total hits were retrieved. For example: Your search has returned 30 out of 30595 hits. You can merge just the results that have been returned, or if you want ZPORTAL to keep returning hits, you can merge the total number of hits up to a maximum of 1000.
After you've merged duplicates, your result list shows you which records had duplicates. Clicking on Details (or clicking the title link) allows you to see the duplicates that were merged.
Generally speaking, records containing the same unique identifiers (such as ISBN, ISSN, LCCN, etc.) are considered duplicates. However, the definition of a duplicate may be determined by your library.